FIG. 7 is a block diagram illustrating a configuration example of a relay device 900 related to the present invention. The relay device 900 is a repeater (optical repeater) for an optical signal, which is used in an optical submarine cable system. The relay device 900 includes optical amplifiers 901 and 902. The optical amplifiers 901 and 902 are designed in such a way as to be suitable for amplification of a C-band optical signal. Note that in the present patent application, the term “C-band” indicates a wavelength band approximately ranging from 1530 nm to 1565 nm.
From an uplink input (UP IN) in FIG. 7, an uplink-channel optical signal whose carrier wavelength is within the C-band is wavelength-multiplexed and input. The input uplink-channel optical signal is amplified by the optical amplifier 901. The uplink optical signal amplified by the optical amplifier 901 is output from an uplink output (UP OUT). From a downlink input (DOWN IN) in FIG. 7, a downlink-channel optical signal whose carrier wavelength is within the C-band is wavelength-multiplexed and input. The input downlink-channel optical signal is amplified by the optical amplifier 902. The optical signal amplified by the optical amplifier 902 is output from a downlink output (DOWN OUT). Optical signals input and output in the relay device 900 are transmitted between the relay device 900 and a land device or another relay device.
At the outputs of the optical amplifiers 901 and 902, signal loopback circuits 903 and 904 are provided respectively. The signal loopback circuits 903 and 904 are each constituted of an optical fiber grating and two optical couplers. The signal loopback circuits 903 and 904 reflect monitoring signals among optical signals output from the optical amplifiers 901 and 902 respectively, and loop back the reflected signals to channels in the opposite directions. The monitoring signal is a signal for monitoring a system and has a wavelength different from that of data to be transmitted. The monitoring signals looped back by the signal loopback circuits 903 and 904 are looped back to land devices by using opposite optical channels. The land device can remotely monitor an operation of the relay device 900 by using the looped-back monitoring signal. For example, a downlink-channel monitoring signal included in an optical signal output from the optical amplifier 901 is combined to an uplink-channel optical signal by the signal loopback circuit 904. As a result, the land device connected to the uplink output (UP OUT) can receive the downlink-channel monitoring signal.
In relation to the present invention, PTL 1 describes an optical amplification relay system including an optical loopback circuit for a monitoring signal.